The present invention generally relates to telecommunications or telephone exchange line circuits and, in particular, relates to a circuit for reducing power dissipation and implementing the basic line drive functions.
With the advent of modern telecommunications systems, there has been many innovations in the design and fabrication of the line circuit. Essentially, a line circuit is employed to interface with each subscriber, whether it be a telephone subset or a PBX trunk interface. The primary functions of the line circuit are to enable the switching system to service each line to determine whether or not the line is on or off hook, whether to apply ringing to the line circuit as well as controlling the level of speech or data that is transmitted or received by the line circuit during a connection.
Due to these factors, a large switching system will, of course, have a large number of line circuits, Modern switching systems such as the ITT 1240 Digital Exchange employ improved types of switching elements and, further, utilize distributed control techniques. Such systems utilize a great deal of software programming to implement system functions. A major cost factor in such switching systems is the line circuit. The line circuit is an extremely important element of any switching system as it is the primary interface with the subscriber. It is apparent that, based on the large number of circuits employed, any reduction in power or any improvement in operation substantially enhances the performance of the entire system. As such, there exists many patents which relate to improved techniques and structures for implementing line circuit functions and in implementing such functions in the above noted type of system.
For example, reference is made to U.S. Pat. No. 4,315,106 issued on Feb. 9, 1982 entitled APPARATUS REGULATING CURRENT SUPPLY TO A TELEPHONE LINE SIGNAL OF THE TYPE EMPLOYED IN DIGITAL TELEPHONE SYSTEMS by R. C. W. Chea, Jr., and assigned to International Telephone and Telegraph Corporation, the assignee hereof. This patent describes a circuit for supplying a regulated loop current to a telephone line.
Also see U.S. Pat. No. 4,317,963 entitled SUBSCRIBER LINE INTERFACE CIRCUIT UTILIZING IMPEDANCE SYNTHESIZER AND SHARE VOLTAGE SOURCE FOR LOOP CURRENT REGULATION CONTROL issued on Mar. 2, 1982 to R. C. W. Chea, Jr. and assigned to the assignee hereof. This patent describes a subscriber line interface circuit where current is supplied to a subscriber line from a shared voltage source such as a DC/DC converter that is regulated to permit a regulated line current to be supplied to many lines from the shared voltage source.
Other patents relating to such improvement are U.S. Pat. No. 4,387,273 entitled SUBSCRIBER LINE INTERFACE CIRCUIT WITH IMPEDANCE SYNTHESIZER issued on June 7, 1983 to R. C. W. Chea, Jr. as well as U.S. Pat. No. 4,349,703 entitled PROGRAMMABLE RING SIGNAL GENERATOR issued on Sept. 14, 1982 to R. C. W. Chea, Jr., both of which are assigned to the assignee hereof.
As one can ascertain, there has been a continuous effort to improve the operation of the line circuit in regard to many pertinent factors.
As indicated, the line circuit is expected to perform many basic line drive functions. Thus the line circuit, apart from supplying talk battery, must be capable of handling ringing signals, voice signals as well as remote metering and testing signals. The problem of ringing, for example, is a substantial problem based on present requirements primarily because the application of a ringing signal to a line circuit requires the use of high voltage commutating switches. Thus, the ringing signal in conventional circuits requires components capable of withstanding high voltage. Such components are not easily integrated and constitute an expensive portion of the line circuit. These components include high voltage isolation transformers, optical couplers, high voltage transistors and other devices.
A major problem in regard to previous line circuits is the fact that the circuits dissipated power that was necessary for line supervision. As indicated, any reduction of power in a line circuit results in a substantial reduction of power in the overall system due to the large number of such circuits.
In addition, the line circuit must be extremely reliable based on the operating environment as well as being able to withstand various surges and transients such as lightning and so on. It has been a major object of work to date to devise a completely integrated line circuit. In this manner, once the basic design has been formulated, the entire line circuit would be integrated in an integrated circuit array which, based on known techniques, can be mass-produced at an extremely reasonable cost.
However, the above noted problems and requirements concerning the basic line driving functions has prevented a complete integration of the line circuit.